Person walks past a Musalee

The Musalee (person reading Salah) had nowhere else to read his Salah besides that particular spot and the person who walked past him could have used an alternate route. In this situation the person who walked past the Musalee will be sinful.

The Musalee could have read his Salah at another place where people would not be passing by but chose to read in a place where the person who walked past him had no alternative routes, in this situation the Musalee would be sinful.

The Musalee and the passer by had alternative places to read Salah and pass by respectively but chose not to. In this instance, both will be sinful. (Aapke Masaail p.416 v.2)